1. Field of the Invention
The present invention relates to an image-forming apparatus employing a so-called reversal developing system. More specifically, the invention relates to an image-forming apparatus which employs a reversal developing system preventing the occurrence of image spots and featuring an improved transfer efficiency.
2. Description of the Prior Art
An image-forming apparatus employing a reversal developing system has heretofore been known. In this reversal developing system, a photosensitive material is uniformly charged into positive or negative polarity and is exposed to image-bearing light such as a laser beam or the like. Then, by attenuating the light, the portion irradiated with light corresponding to the document image is rendered to possess a residual potential of from 0 V to 100 V to form an electrostatic latent image. Then, a toner electrically charged to the same polarity as the polarity of the electrically charged photosensitive material is brought into contact with the photosensitive material followed by developing. The toner adhered onto the portions of potentials of from 0 V to 100 V on the surface of the photosensitive material is then transferred onto a transfer material such as paper or the like to form the image.
According to the above-mentioned image-forming method based on the reversal developing system, the toner image formed on the surface of the photosensitive material is transferred onto a paper or the like by using a transfer roller to which is applied a DC voltage of a polarity opposite to that of the toner image. In a step of transfer, therefore, there remains a problem in that the surface of the photosensitive material may be electrically charged into an opposite polarity, the electric charge will not be effectively discharged and the image may not be effectively formed.
In order to eliminate such a defect inherent in the image-forming apparatus employing the reversal developing system, Japanese Laid-Open Patent Publication No. 7086/1989 proposes means in which a DC voltage applied to the transfer roller is set to be smaller than a charge start voltage at which the photosensitive material is started to be electrically charged.
As a transfer roller to be used for such a transfer device, furthermore, Japanese Laid-Open Patent Publication No. 177063/1989 discloses the one which has a hardness of not larger than 30 degrees (JIS A). By using a transfer roller having a small hardness, this transfer device prevents the toner on the surface of the image carrier member from coagulating and enables the surface of the image carrier member to be easily cleaned.
Japanese Laid-Open Patent Publication No. 200277/1989 discloses a transfer system using a transfer roller similar to the one mentioned above, wherein an electric charge of a polarity opposite to that of the toner is fed to a transfer material such as a paper or the like in a state of maintaining a gap between the transfer material and an image carrier (photosensitive material), and the transfer material is then brought into contact with the image carrier.
According to means taught by the above-mentioned prior arts, however, only a DC voltage smaller than a charge start voltage is allowed to be applied to the transfer roller. Therefore, the image is poorly transferred due to a decrease in the transfer efficiency and problems still remain for obtaining good images.
In the image-forming apparatus of the reversal developing system, on the other hand, when a voltage larger than a charge start voltage is applied to the transfer roller, the transfer roller is electrically charged to a polarity opposite to that of the main charge of the photosensitive material. The electric charge of this opposite polarity cannot be discharged in the discharging step that precedes the step of electrophotography. During the step of main charging, therefore, there develop portions having low potentials due to offset of the potential, resulting in the development of potential spots, i.e., image spots. The image spots are undesirable even in a line image and become particularly conspicuous in the case of a half-tone image.
The method using the transfer roller having a small rubber hardness is effective in preventing the photosensitive material from being worn out and in preventing the toner from coagulating. However, a decrease in the hardness of the rubber results in the occurrence of problems such as thermal deformation of the rubber and deformation due to heat and aging, making it difficult to maintain the contact between the transfer material and the photosensitive material constant at all times. When a gap larger than the thickness of the transfer material is provided between the transfer roller and the photosensitive material, the effect of transfer due to the surface state of the transfer roller surely decreases. However, the supply of current necessary for transferring the toner loses stability and the image becomes subject to be affected by environment such as humidity and the like.